The difference is that most people don't write their business logic in a single flat function in most programming languages, and there is a good chance that individual pieces would have tests.
Neither do good SQL developers: you can easily structure database logic in multiple stored functions, stored procedures, packages, and multiple files. Unit test frameworks are also available.
Testing SQL is pretty popular in most pro-SQL environments as well. There are many tools for that even if many developers appear to remain willfully ignorant of them.
